1. Field of the Invention
The present invention relates to a knob for a thermostatic control that has positioning elements having portions that may be modified to compensate for variations in the temperature at which the bimetallic element opens and a method of providing an adjustable thermostatic control for a heating appliance.
2. Background Art
Household appliances and other devices having thermostatic heat controls are widely used in devices such as water baths, paraffin baths, electric heaters, and other household and kitchen appliances. The heat control for an appliance may include a thermostatic element comprising a bimetallic strip having a contact that opens the circuit and interrupts current supplied for heating the appliance to a desired temperature.
Such bimetallic thermostat elements may be obtained that are capable of sensing temperatures with different degrees of accuracy. Generally, more precise bimetallic elements can provide heat control within a few degrees or even fractions of degrees. Generally, more precise or limited temperature range bimetallic thermostatic elements will be more expensive. To provide cost effective thermostatic control for household appliances, the use of bimetallic elements having a temperature tolerance of plus or minus 8-10xc2x0 F. has been proposed. By way of example, bimetallic thermostatic elements having a temperature specification of 128xc2x0 F. and a tolerance specification of plus or minus 8xc2x0 F. were unsatisfactory for use in paraffin baths because the thermostats would permit temperatures to range from 120xc2x0 F. to 136xc2x0 F. Use of such broad tolerance thermostats was considered unacceptable since at the low end of the temperature range the paraffin wax is slow to melt while at the upper end of the temperature range the bath would be uncomfortable to a user. The ideal temperature range of between 126xc2x0 F. and 131xc2x0 F. was not possible to assure with broad tolerance bimetallic elements. While more expensive lower tolerance thermostatic elements could be specified they would tend to add substantially to the cost of the appliance.
What is needed is a method and apparatus for providing thermostatic heat control for a household appliance utilizing broad tolerance bimetallic thermostatic elements while providing heat control within the more narrow range than the range of potential tolerance of the bimetallic thermostatic element.
These and other problems shortcomings to the prior art are addressed by Applicant""s invention as summarized below.
According to the present invention, a thermostatic control is provided for heating an appliance having an electrical circuit for controlling the heating appliance including a bimetallic element. A potentiometer is provided in the circuit for selecting the temperature level within an acceptable range of temperatures. The potentiometer has a rotatable shaft having a first positioning element. A knob is secured to the shaft and has a second positioning element that engages the first positioning element. The first and second positioning elements have portions that may be modified to compensate for variations in the temperature at which the bimetallic element opens.
According to another aspect of the invention, the first positioning element on the rotatable shaft may be a rib and the second positioning element may be a tubular portion of the knob having a slot that receives the first positioning element. Further, the slot of the second positioning element may be located within a range of radial locations. For example, three different types of knobs may be provided with the slot being located in one of three radial locations to compensate for variations in the temperature at which the bimetallic element opens.
According to another aspect of the invention, the heating appliance may include a housing with at least one fixed stop while the knob has at least one end stop that cooperates with the fixed stop to limit rotational movement of the rotatable shaft.
According to another aspect of the invention, the heating appliance may have a control panel having an opening through which the knob is connected to the rotatable shaft. The knob may have projections including detents that rotatably secure the knob to the opening in the control panel.
According to another aspect of the invention, the bimetallic element may have a rated temperature tolerance within a first range of temperatures. If the range of acceptable temperatures for the heating appliance is less than the rated temperature tolerance, the portions of the first and second positioning elements may be selected or arranged to calibrate the potentiometer to fall within the acceptable range of temperatures.
According to yet another aspect of the invention, a two-part knob may be provided including an outer part defining a cavity and an inner part comprising a disk that is received in the cavity. The outer portion may include a tubular portion and the inner portion may have two end stops and two projections including detents for securing the knob to a control panel.
According to the method of the present invention, an adjustable thermostatic control may be provided for a heating appliance that comprises testing a bimetallic element having a contact that opens at a temperature within a range of temperatures. Based upon the testing, the bimetallic elements are sorted into at least two groups depending upon the point within the range of temperatures that the contact opens. A first manual control part that cooperates with a second manual control part is selected that has positioning elements that may be selected to compensate for variations in the temperature at which the bimetallic element opens by setting the location that the first and second manual control parts engage each other.
According to another aspect of the invention, the bimetallic elements may be sorted into three groups including a low range, mid-range, and high range temperature groups.
According to yet another aspect of the method of the present invention, the first manual control part is a knob having a slot formed in a tubular portion thereof and the second manual control part is a rotatable shaft having a rib that is received in the slot. The knob may be made with a slot in different radial positions relative to end stops to establish the location that the first and second manual control parts engage each other.
These and other objects, features, and advantages of the present invention will be better understood in view of the attached drawings and following detailed description of the preferred embodiment.